System and method for coordination of wireless maintenance channel power control

ABSTRACT

In a wireless communication system, wireless channels are maintained for communication between users and the base station. Often, a wireless user may be switched on, but not actively sending or receiving data. Accordingly, wireless users may be “active,” and currently allocated a wireless traffic channel for sending or receiving, or “idle,” and not currently sending or receiving. A wireless user may be maintained in an idle state through a periodic sequence of synchronization messages. A method for controlling a power level of a wireless message includes determining the presence of data to be transmitted, and adjusting the power level of the synchronization message depending on the presence of data. The synchronization messages corresponding to the idle state employ a lower power level than the active state transmissions which employ a higher power level. The system therefore controls the power accordingly such that synchronization messages are sent at a lower power level when no data is present, thereby reducing power consumption and interference.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 10/170,015, filed on Jun. 11, 2002 which claims the benefit of U.S. provisional application No. 60/297,839 filed on Jun. 13, 2001, which is incorporated by reference as if fully set forth.

BACKGROUND

In a wireless communication system, a number of radio channels provide a connection between users and a central location, such as a base station or access point. In such a system, the wireless channels are a scarce resource which must typically be shared. In a Code Division Multiple Access (CDMA) system, a number of different channels can be transmitted on a single radio frequency carrier by applying different codes to each signal. However, even in a CDMA system, demand for access to channels is so great that the base station must allocate and switch the channels among multiple users.

Often, a wireless user may be switched on, but not actively sending or receiving data. Accordingly, wireless users may be in an “active” mode, and currently allocated a wireless data traffic channel for sending or receiving data, or in an “idle” mode, and not currently sending or receiving data. An idle user may, for example, have just sent or received a data traffic transmission and is therefore deemed likely to soon request a data traffic channel for further transmissions. A maintenance message may therefore be employed to maintain a user in a synchronized but idle state to facilitate allocation of a wireless traffic channel when needed. When a user requests a channel, the idle state allows the user to be allocated a wireless traffic channel more quickly than for a user who was not being maintained in a synchronized idle state. For more information concerning one way to implement a system, please refer to U.S. Pat. No. 6,222,832, entitled “Fast Acquisition of Traffic Channels for a Highly Variable Data Rate Reverse Link of a CDMA Wireless Communication System,” (Attorney's Docket No. 2479.1008-000) assigned to Tantivy Communications, Inc., assignee of the present application.

A number of users, therefore, may be maintained in an idle state through a periodic sequence of maintenance messages. In the idle state, the maintenance messages typically provide time tracking and power control, and do not require phase reference information employed when in the active state. Time tracking and power control signaling requires less power than the maintenance messages sent during active data payload transmission. The maintenance messages, however, are typically sent at a similar power level during both the idle and active states. Accordingly, the maintenance messages can increase interference and battery power drain during the idle state.

SUMMARY

In a wireless communications system, synchronization maintenance messages are often employed to maintain a user in an idle state by providing time tracking and power control. According to the present invention, a method for controlling the power level of a wireless message which defines a maintenance channel operable to transmit synchronization maintenance messages (synchronization messages) includes determining the presence of data to be transmitted from a wireless access terminal to a base station. The power level of the synchronization message sent from the access terminal via the maintenance channel is adjusted depending on the presence of data to be sent. Synchronization messages for idle state synchronization provide time tracking and power control signaling, while synchronization messages corresponding to active data traffic transmissions also provide phase reference for the data traffic transmissions. The synchronization messages corresponding to the idle state employ a lower power level than the active state transmissions which employ a higher power level.

In this manner, the system monitors the presence of data and controls the power level accordingly such that synchronization messages are sent at a lower power level in the idle state, when no data is present, thereby reducing power consumption and interference.

More specifically, a data transmission state is maintained at the wireless access terminal to indicate the presence of data to transmit via a data traffic channel on a reverse link. The power level of a synchronization message is computed in response to the data transmission state. Target power levels are maintained for the idle state and the active state. The synchronization messages are sent from the access terminal to the base station at the corresponding power level. Power control messages (return messages) sent in response from the base station manage the power level towards the applicable target power level.

A maintenance channel connection is maintained for transmission of the synchronization messages. The maintenance channel typically transmits unmodulated, or pilot, signals for maintaining synchronization. Since the maintenance channel is not a dedicated data traffic channel, a plurality of access terminals may be maintained over a single maintenance channel using a plurality of time slots.

The synchronization messages are typically sent at predetermined intervals from each of the access terminals. In the idle state, synchronization messages are sent according to a gating rate. In the active state, synchronization messages are sent continuously in order to maintain a phase reference for a corresponding data traffic transmission. Return power control messages are sent in response to the synchronization messages including power control and time tracking information according to a power control group.

The access terminal determines the data transmission state, either ON (active) or OFF (idle), and adjusts transmission power accordingly. The base station, in turn, determines the data transmission state and adjusts information in the return messages accordingly to control the power level toward one of an active power control target and an idle power control target. Further, the access terminal disregards return messages for a predetermined interval after a change in the data transmission state to allow for recognition and adjustment of the data transmission state change by the base station. The target power level is determined by the base station using factors including the received signal strength, received signal quality, Carrier-to-Interference (C/I) ratio, and the Signal to Noise Ratio (SNR).

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIGS. 1 a-1 c are diagrams of prior art maintenance messages for a wireless maintenance channel;

FIG. 2 is a diagram of a wireless communication system employing the invention as defined by the present claims;

FIG. 3 shows the forward and reverse links between a wireless access terminal and a base station for message transmission;

FIG. 4 is a diagram of wireless synchronization messages as defined by the present claims;

FIG. 5 shows idle and active power levels;

FIG. 6 is a flowchart of message transmission;

FIG. 7 is a flowchart of a power control loop for managing transmission power according to the target power levels; and

FIG. 8 is a message transmission sequence diagram showing the transition between data transmission states.

DETAILED DESCRIPTION

A description of preferred embodiments of the invention follows.

The wireless system as disclosed herein employs a reverse link maintenance channel for maintaining synchronization and other state information for a plurality of subscriber access terminals. The subscriber access terminals support users by providing a wireless link to a base station processor for communication with a data network such as the Internet. The wireless link is provided by one or more wireless channels managed by the base station. The wireless channels are dynamically allocated by the base station among the multiple access terminals depending on data transmission needs, and typically do not remain dedicated to a single user.

A maintenance channel, therefore, is employed to maintain an access terminal in synchronization with the base station when it is not actively sending data. Such a maintenance channel is capable of maintaining a plurality of access terminals at the same time. This synchronization allows an access terminal to be allocated a data traffic channel more readily when needed for data traffic transmission than that which would be required to set up and tear down a reverse link wireless channel each time the access terminal was to send or receive data messages.

FIGS. 1 a-1 c show a diagram of several different types of maintenance messages used in the prior art. Referring to FIGS. 1 a-1 c, the power level of the maintenance messages are shown for three different gating rates—1, ½ and ¼, respectively. When an access terminal is in an idle state and not actively sending or receiving data, as shown by the “data off/gated” period 10, a maintenance message is sent to maintain time tracking and power control. The idle state message need not be continuous. It is sent in a time slotted or gated manner according to a gating rate 14 a-14 c, for the duration of a 20 ms power control group 18. As shown in FIGS. 1 a-1 c, respectively, several different gating rates have been used. During a “data on” period 12, the maintenance message is sent continuously, as shown by the continuous transmissions 16 a-16 c. However, during each power control group 18 gated messages are sent at the same power level, as shown by the power level on axis 20. As the maintenance messages sent during the data off period 10 are employed for time tracking and power control, they need not be sent at the same power level afforded to the maintenance messages sent during the data on period 12, which are used also for phase reference for a corresponding data traffic channel.

FIG. 2 is a diagram of a wireless communication system 22 employing the invention as defined by the present claims. A plurality of subscriber terminals 24 (access terminals) are in wireless communication with a base station processor 26 (base station) via wireless links 30. The base station 26 is also connected to the Internet 28 via a wired link 32 to act as a gateway for the access terminals 24. The access terminals 24 provide wireless Internet access to customer premises equipment (CPE) 32 generally, such as desktop PCs 32 a, 32 c, personal digital assistants (PDAs) 32 b, wireless phones 32 d, and other devices such as laptops, pagers, and automotive telematics devices, not shown. It should be noted that the wireless functionality provided by the access terminal 24 may be in a stand alone device such as a subscriber access unit or embedded in the CPE 32 unit. In either case the CPE is operable to communicate with the Internet 28 via the wireless link 30 and the base station 26.

FIG. 3 shows forward and reverse links between an access terminal and a base station for message transmission. Referring to FIGS. 2 and 3, the wireless links include both a forward link 34 and a reverse link 36 to permit duplex communication. The forward link supports wireless channels for carrying messages from the base station 26 to the access terminal 24, and the reverse link 36 supports wireless channels for carrying messages from the access terminal 24 to the base station.

Each of the access terminals 24 periodically sends a synchronization message via the reverse link 36 to the base station 26. The synchronization message includes time tracking and power control information sufficient to maintain the access terminal in at least an idle state, and therefore synchronized with the base station 26. The base station responds with a power control message via the forward link 34. The power control message includes power control commands to direct the power level of subsequent messages to allow the access terminal 24 to remain synchronized with the base station 26. The power level is determined by a power level controller 38 at the base station for computing a target power level for both the idle and active data transmission states.

The forward 34 and reverse 36 links further comprise data traffic channels for transmitting wireless messages having a data payload. The data traffic channels are allocated by the base station 26 to an access terminal 24 when the access terminal 24 is to send or receive data. A data transmission state, described further below, is indicative of whether the access terminal 24 is allocated a data traffic channel. When the data transmission state is active, the synchronization messages provide a phase reference for the messages transmitted on the data traffic channels, in addition to time tracking and power control.

Synchronization messages employed for only for time tracking synchronization and power control, however, do not require as much power as the synchronization messages used to provide phase reference during the active data transmission state. Therefore, according to the present invention, synchronization messages are sent at a reduced power level in the idle data transmission state. Additional power is provided for the synchronization message when it is also employed for phase reference. Accordingly, the synchronization messages are sent at a lower power level in the idle data transmission state than in the active data transmission state.

FIG. 4 is a power level diagram for the invention. Referring to FIGS. 4 and 2, during an idle (OFF) data transmission state 40, the synchronization messages are sent in a gated manner according to a gating rate. A gating rate of ¼ is shown as exemplary; other gating rates could be employed. The power level of the message is shown by the axis 44. When the data transmission state transitions to active (ON) 42, the power level 44 is increased to allow the synchronization messages to be employed for phase reference as well.

As indicated above, synchronization messages are transmitted at one of two power levels. The power level is managed by the base station 26 and transmitted to the access terminal 24 via the power control messages. The access terminal 24 responds by transmitting at the power level proscribed by the base station. The base station 26 computes a target power level, described further below, indicative of the power level at which the access terminal should transmit. Typically, the power level is expressed as effective radiated power in decibels (dB), however, other metrics could be employed. The base station, therefore, maintains target power levels for each of the data transmission states. An active target power level corresponds to the ON data transmission state and an idle target power level corresponds to the OFF data transmission state.

FIG. 5 shows idle and active power levels in more detail. Periods 46 correspond to OFF data transmission state, and period 48 corresponds to ON data transmission state. The power level 44 indicates the power level of the synchronization messages. The idle target power level, shown by dotted line 50, indicates the power level at which synchronization messages should be sent during OFF data transmission state. The active target power level, shown by dotted line 52, indicates the power level at which synchronization messages should be sent during ON data transmission state.

The access terminal 24 maintains the idle and active target power levels 50, 52. The base station 26 manages the transmission power of the messages sent by the access terminal 24 by the power control messages, which are sent by the base station 26 to adjust the idle and active target power levels 50, 52. The access terminal 24 determines when the data transmission state changes and toggles between the active and idle transmission power levels, and transmits according to the corresponding power level. The base station 26 determines a change in the data transmission state, described further below, and adjusts the power control messages accordingly.

While the data transmission state affects whether transmission occurs at the active or idle power level, other factors affect the perceived power level as well. The distance from the access terminal 24 to the base station 26, intervening objects, interference from other sources, and other factors all can affect the perceived power level of the wireless messages. Accordingly, the base station 26 examines the received signal quality, indicative of the power level of a received message, and computes the power control message accordingly. If a message from the access terminal 24 is being received at too low a perceived power level, the base station will transmit power control messages indicative of a higher power level at which to transmit. Similarly, if a message is received from the access terminal 24 at too high a perceived power level, the base station will transmit power control messages indicative of a lower power level at which to transmit. The base station, therefore, manages the power level of messages transmitted from the access terminal by focusing on a target power level.

Accordingly, when the access terminal 24 changes data transmission states, the base station will receive messages at a different power level. The base station determines that the change in power level is due to a change in data transmission state, and not to other factors described above, and continues to compute the idle and active power levels accordingly. Further, as the power control messages are typically sent according to power control groups of 16 every 20 ms, or every 1.25 ms, the base station may not determine the data transmission state until several power control message cycles. Accordingly, the access terminal 24 may disregard power control messages for a predetermined period after toggling data transmission states. The access terminal, therefore, may toggle between the active and idle power levels without the base station 26 countering with power control messages which would otherwise undermine the transmission of reduced power synchronization messages.

FIG. 6 is a flowchart of synchronization message transmission according to a particular embodiment of the invention. Referring also to FIG. 2 again, an access terminal 24 determines if there is data ready to be transmitted on a traffic channel, as depicted at step 100. A check is performed to set or maintain the data transmission state accordingly, as shown at step 102. If there is no data waiting to be transmitted, the access terminal 24 enters or maintains the data transmission state of idle, as depicted at step 104. If there is data waiting to be transmitted, the access terminal 24 enters or maintains the data transmission state of active, as shown at step 106.

In the idle data transmission state, the access terminal sets 24 the transmission power level at the idle target power level, as disclosed at step 108. The access terminal then determines the gating rate of the idle state message, as determined at step 110. In the idle mode, the message is sent in a gated, or periodic manner, such as ¼, ½, or 1. The gating rate results in a periodic delay, prior to sending the next synchronization message.

In the active data transmission state, the access terminal 24 set sets the transmission power level at the active target power level, as shown at step 112. The access terminal then sets the message as continuous, with no gating, as shown at step 114.

The access terminal 24 then sends a synchronization message to a base station 26, as depicted at step 116. The base station 26 receives the synchronization message, as disclosed at step 118, and determines the data transmission state, as shown at step 120.

Determination of the data transmission state is discussed further below with respect to FIG. 7. A check is performed to examine the determined data transmission state, as disclosed at step 122. If the data transmission state is idle, then the base station computes or maintains a new idle target power level, as depicted at step 124. If the data transmission state is active, then the base station computes a new active target power level, as shown at step 126. The base station 26 then sends a power control message indicative of the computed target power level to the access terminal 24, as shown at step 128.

The access terminal 24 receives the power control message, including the new target power level, as shown at step 130. The access terminal 24 then determines when to send the next synchronization message, depending on gating rate, as depicted at step 132, and control reverts to step 100.

In a gated manner, the access terminal 24 periodically sends the synchronization message depending on the gating rate. Accordingly, the access terminal 24 may wait for one or more power control group intervals of 1.25 ms each to elapse before sending the next synchronization message, as shown above in FIG. 4. Alternatively, in the active data transmission state, the synchronization messages are sent in a continuous manner, also as shown in FIG. 4.

FIG. 7 is a flowchart of a power control loop for managing transmission power according to the determined target power levels. Referring to FIG. 2 as well, the determination of the data transmission state is shown. The base station 26 receives the synchronization message from the access terminal 24, as shown at step 150. The base station determines which metric to employ to determine the data transmission state at the access terminal 24, as shown at step 152. As the access terminal toggles the data transmission state between active and idle, the base station determines the current data transmission state from the synchronization messages as they are received. The base station 26 then attempts to set the target power level reflected in the power control messages accordingly.

The base station 26 examines the received quality level of the synchronization message to determine the power level at which it was sent, and hence the data transmission state of either idle or active, as depicted at step 154. As indicated above, the access terminal 24 transmits at one of the idle or active power levels depending on the data transmission state. However, the base station 26 attempts to adjust the target power level such that access terminal transmissions are received uniformly at the base station 26. Accordingly, the base station 26 determines a transmission quality based on a link quality metric. Since the received quality level may be affected by other factors in addition to transmission power employed by the access terminal 24, such as noise, interference, and reflection, other metrics may be used to assess received signal quality and determine the sending power level.

Alternatively, the base station may receive a separate data transmission state indication according to a periodic interval, as shown at step 156. Such a predetermined interval may be according to the gating rate, or according to a predetermined interval agreed to by the base station and the access terminal, as described in copending U.S. patent application Ser. No. 60/346,527, entitled “Coordination of Pilot Power Level Based on Periodic Intervals” (Attorney's Docket No. 2479.2141-000), filed Jan. 8, 2002 and incorporated herein by reference.

The base station 26 may also receive the data transmission state encapsulated in the synchronization message, as shown at step 158. A detectable signal encapsulated in the synchronization message is detected by the base station, and employed to set the data transmission state, and the associated target power control level, at the base station 26 accordingly.

In another embodiment, the base station employs a MAC (media access control) state to determine the data transmission state, as disclosed at step 160. The MAC state transitions are indicated by a signal in the synchronization message. The base station detects the MAC state change, as described in copending U.S. patent application Ser. No. 60/346,525, entitled “Coordination of Pilot Power Level Based on MAC State” (Attorney's Docket No. 2479.2140-000), filed Jan. 8, 2002, incorporated herein by reference, and sets the data transmission state accordingly.

In alternate embodiments, other methods may be employed to detect a change in data transmission state at the base station 26. The base station 26 therefore, determines the data transmission state from the synchronization message, and sets its own indicator accordingly, as depicted at step 162. The indicator is employed in determining the target power level to set in the power control message, as disclosed at step 164. Other factors which affect the target power level include the C/I (Carrier to Interference) ratio or the SNR (Signal-to-Noise ratio), in addition to the received power level. Further discussion of power control messages and power control groups is disclosed in copending U.S. patent application Ser. No. 09/999,172, filed Nov. 30, 2001, entitled “Antenna Control System and Method” (Attorney's Docket No. 2479.2065-002), incorporated herein by reference. Once the target power level is determined, corresponding to a data transmission state of idle or active at the access terminal 24, control resumes at step 120 in FIG. 6.

FIG. 8 is a message transmission sequence diagram showing a transition between data transmission states. A timing diagram 60 of a maintenance channel is shown. A plurality of synchronization messages 62 a-62 g are sent from an access terminal 24 to a base station 26. A complementary plurality of power control messages 64 a-64 g are sent from the base station 26 to the access terminal 24. The data transmission state (DT) at the access terminal 24 is shown by the hatched bars 66, and the DT state at the base station are shown by the hatched bars 68. At an initial time T=0, the DT state 66 at the access terminal 24 is idle (I), as shown by hatched area 70, and the DT state at the base station 26 is also idle for this user, as shown by hatched area 72. The synchronization message 62 a is sent at a power level (PL) corresponding to the idle power level, and is sent in a gated (G=Y) manner since only synchronization information need be transmitted. The base station 26 responds with a power control message 64 a indicating power control (PC) is to be maintained at the Idle level. The access terminal 24 receives the power control message 64 a to maintain the Idle power level. The next synchronization message 62 b is sent, also PL=I and G=Y, and the base station 26 responds with power control message 64 b, PC=I. A series of idle state (DT=I) messages may continue, described above with respect to FIGS. 6 and 7, as shown by dotted line 90.

The access terminal 24 detects data present to be transmitted on a data traffic channel (not shown), and changes the DT state 66 to active (ON), as shown by hatched area 74. The synchronization message 62 c is now transmitted at a power level of “A” (Active) and is sent in a constant (non-gated) manner G=N. Upon receiving the synchronization message, the base station 26 determines that there is data traffic present, and toggles the DT state 68 to A, as shown by hatched bar 76. The base station responds with power control message 64 c, indicating power control is to target the active level (PC=A). The synchronization message 62 d is transmitted at PL=A and G=N, and a power control message 64 d is received for PC=A. A series of active state (DT=A) messages may continue, as described above with respect to FIGS. 6 and 7, as shown by dotted line 92. Synchronization message 62 e is then sent, which corresponds to the last active state message in this sequence 62 c-62 e.

Power control message 64 e is sent, and the access terminal 24 determines that there is currently no more data to transmit. Accordingly, the DT state 66 toggles to “I,” (Idle) as shown by hatched area 78, and synchronization message 62 f is sent, at PL=I and G=Y. The base station 26 receives the message 62 f, determines that there is no data presently being transmitted, and toggles the DT state 68 to “I,” as shown by hatched area 80. Power control message 64 f is sent in response, at PC=I. Idle mode messages 62 g and 64 g follow in order, to continue the synchronization maintenance cycle, until the next active DT state.

The embodiment described above includes two power control levels of idle and active as illustrative. Multiple power level thresholds could be maintained between a access terminal 24 and a base station 26. Accordingly, the invention as described herein may be employed to provide multiple levels of standby or idle status, depending on the level of signaling capability employed at each level, for the purpose of minimizing interference and maintaining synchronization between an access terminal 24 and a base station 26.

Those skilled in the art should readily appreciate that the system and methods for synchronization message power control as defined herein are deliverable to a wireless device in many forms, including but not limited to a) information permanently stored on non-writeable storage media such as ROM devices, b) information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media, or c) information conveyed to a computer through communication media, for example using baseband signaling or broadband signaling techniques, as in an electronic network such as the Internet or telephone modem lines. The operations and methods may be implemented in a software executable by a processor or as a set of instructions embedded in a carrier wave. Alternatively, the operations and methods may be embodied in whole or in part using hardware components, such as Application Specific Integrated Circuits (ASICs), state machines, controllers or other hardware components or devices, or a combination of hardware, software, and firmware components.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 

1. A method implemented in a subscriber terminal, the method comprising: transmitting a synchronization message on a reverse link, wherein the synchronization message includes a phase reference for a corresponding data channel; receiving a power control message from a base station in response to the synchronization message; and adjusting the power level of a subsequent synchronization message based on the received power control message.
 2. The method of claim 1 wherein the phase reference in the synchronization message provides coherent demodulation of the data channel.
 3. The method of claim 1 wherein communication is performed via a code division multiple access (CDMA) network.
 4. A subscriber terminal comprising; a receiver configured to a receive a power control message; a processor configured to adjust a power level of a synchronization message based on the received power control message; and a transmitter configured to transmit the synchronization message on the reverse link, wherein the synchronization message includes a phase reference for a corresponding data channel.
 5. The subscriber terminal of claim 4 wherein the phase reference in the synchronization message provides coherent demodulation of the data channel.
 6. The subscriber terminal of claim 4 wherein communication is performed via a code division multiple access (CDMA) network. 